1. Field of the Invention
This invention relates in general to improvements in an air-fuel ratio feedback control system for an internal combustion engine provided with an evaporative emission control device, and more particularly to such an air-fuel ratio feedback control system provided with means for preventing temporary air-fuel ratio fluctuation of air-fuel mixture to be inducted into the engine, due to purge of fuel vapor absorbed in the evaporative emission control device.
2. Description of the Prior Art
An Electronically controlled fuel injection system for an automotive internal combustion enigne includes a fuel injector valve which is such arranged as to open in response to drive pulse signal produced in timed relation to engine speed of the engine, so that fuel at a predetermined pressure is injected during a time period of opening of the fuel injector valve. Accordingly, the amount (fuel injection amount) of fuel to be injected from the fuel injector valve is controlled in accordance with the pulse width of the drive pulse signal. Assuming that Ti is the pulse signal corresponding to the fuel injection amount, Ti is calculated to obtain the stoichiometric air-fuel ratio as a tagrget air-fuel ratio, by the following equation: EQU Ti=Tp.multidot.COEF.multidot..alpha.+Ts
where Tp is fundamental pulse width corresponding to fundamental fuel injection amount and referred to as fundamental fuel injection amount. The fundamental fuel injection amount is calculated by an equation of Tp=K.multidot.Q/N where K is a constant, Q is the amount of intake air inducted to an engine; N is the engine speed of engine. COEF is various correction coefficients such as engine coolant temperature correction and acceleration correction coefficients;.alpha. is an air-fuel ratio feedback correction coefficient for feedback control of air-fuel ratio discussed after; and Ts is a voltage correction amount for correcting the fuel injection amount variation in the fuel injector valve, due to fluctuation in voltage of a battery.
The air-fuel ratio feedback control is carried out such that actual air-fuel ratio of the air-fuel mixture is detected by an oxygen (O.sub.2) sensor thereby to judge as to whether the air-fuel ratio is rich or lean relative to stoichiometric air-fuel ratio. In this regard, the above-mentioned feedback correction coefficient .alpha. is determined and controllably varied in order to converge the air-fuel ratio into the stoichiometric value. Here, the value of the air-fuel ratio correction coefficient .alpha. is varied by a proportional-plus-integral control (PI control) thereby to accomplish a stable control thereof. More specifically, in this control, the output voltage of the oxygen sensor is compared with a slice level voltage. In case the output voltage of the oxygen sensor is higher or lower than the slice level, the air-fuel ratio is prevented from being suddenly made richer or leaner. That is to say, when the air-fuel ratio is rich ( or lean) relative to the stoichiometric air-fuel ratio, the air-fuel ratio feedback correction coefficient .alpha. is first reduced (or increased) by the amount of a predetermined proportion constant P and then gradually reduced (increased) by the amount of a predetermined integration constant I, thus appoaching the air-fuel ratio to the stoichiometric value. Such a control is disclosed, for example, in Japanese Patent Publication No. 54-25973.
Here, feedback control constants such as the above-mentioned proportion and integration constants P and I are better to be larger to obtain high follow-up ability for the air-fuel ratio against variation of the intake air amount; however, they are better to be smaller to minimize the width of air-fuel ratio variation during control. Even in this, regard, control or matching is accomplished by employing the latter prior to the former.
Now, most automotive vehicles are equipped with an evaporative emission control device in which fuel vapor generated in a fuel tank is introduced into an activated charcoal canister to be absorbed in activated charcoal, and the thus absorbed fuel vapor is purged with fresh air introduced to the activated charcoal canister under a predetermined engine operating condition. The mixture of the fuel vapor and the fresh air are thereafter sucked through an intake manifold into the engine so that the fuel vapor is combusted in combustion chambers of the engine.
Difficulties have been encountered in such an engine equipped with the evaporative emission control device. When the fuel vapor absorbed in the activated charcoal canister is purged and sucked into the engine, the air-fuel ratio of the air-fuel mixture to be supplied to the engine unavoidably temporarily shifts from the target value, so that the air-fuel ratio becomes far from the target value for a while owing to delay of feedback control. This deteriorates exhaust emission control and driveability of the engine.